I. Field of the Invention
This invention relates generally to electronic sensors for monitoring both temperature changes and sound using a single transducer, and more particularly to the construction of such sensors that more readily allows the temperature signal to be separated from the sound signal using conventional signal processing techniques (filtering).
II. Discussion of the Prior Art
In the Stasz U.S. Pat. No. 5,311,875, which is hereby incorporated by reference, there is described a system for electronically monitoring breathing patterns. The system described therein utilizes a plastic film exhibiting both pyroelectric and thermoelectric properties such as a polyvinylidene fluoride (PVDF) film. The film transducer has a conductive electrode on opposed major surfaces thereof and electrical wires connect the electrodes to an electronics module that is designed to separate the transducer output into two separate channels, one being for temperature and the other being for sound or vibration.
Our testing has shown that the signal proportional to temperature variation greatly exceeds that due to noise or vibration by a factor of about 150:1. The temperature-related signal also exhibits a substantially faster response time than the sound signal and is rich in frequency components in the 20 Hz to 50 Hz range. These facts result in a difficulty in preventing the thermal component of the transducer from crossing over into the sound channel, often leading to the occurrence of false positives when both sound signals and temperature signals are being simultaneously monitored.
The problem thus presents itself as to how one may effectively separate the signals when the frequency components of each signal overlap. Resorting to a signal processing approach has proven difficult.
We have found that by modifying the mechanical construction of the pyro/piezo transducer to increase its thermal mass, the effective rise time of the thermal signal is greatly reduced, while not seriously attenuating the sound/vibration-related signal amplitude. Decreasing the rise time of the thermal signal results in a shift of the frequency components thereof so as not to overlap with the frequency components in the sound channel. As such, conventional filtering steps can be performed to produce separate, isolated channels.